A product such as foods or the like which has been placed in a hermetically sealed container with a space portion and contains protein, saccharide and starch generates carbon dioxide by Strecker degradation. Strecker degradation is an oxidative decarbonizing reaction of protein, saccharide and the like which occurs during retort sterilization and during storage, so that unlike putrefaction due to bacteria or the like, it creates no sanitary problem.
Since the internal pressure of the container is increased with Strecker degradation, in case of a container having a wall portion susceptible to bulging with the enhanced internal pressure, the wall portion swells out and the container is apt to be misconceived as a so-called "swelled container". Since, also when a food in a hermetically sealed container putrefies, the container swells out with generated gases, generally the food in the "swelled container" is presumed to have putrefied.
To avoid this misconception, it is desirable that the internal pressure in the hermetically sealed container prior to the retort sterilization is set to a negative pressure, that is, a pressure lower than the atmospheric pressure, of the degree that there is no danger of being misconceived as the "swelled container", even though Strecker degradation has been generated.
Conventionally, the following methods have been adopted as means for setting the internal pressure in the hermetically sealed container to a negative pressure with the main purpose of removing oxygen harmful to the long-term preservation of food products from the space portion: (a) a method of filling or placing the product and hermetically sealing both in vacuum, (b) a method wherein a stream generated from a boiler is directly blown through a nozzle into a container body placed with the product to substitute the steam for the air in the space portion, and then the container body is hermetically sealed, and (c) a so-called "hot pack method" wherein, immediately after a liquid product, such as juices or the like, heated to about 80.degree. to 90.degree. C. has been placed in the container body, the container body is hermetically sealed.
However, the vacuum method (a) has problems that the apparatus is complicated, the installation cost is relatively high, and the operation efficiency is too low for a high speed production.
In the steam method (b), the controls of the amount and pressure of the blown steam are difficult. Particularly, in case where the steam method is adopted to a container called a semi-rigid container in the description which will be explained hereafter, a very narrowly holed nozzle must be used because the required negative pressure is low (for example, approximately -4 cmHg in gage pressure), so that the nozzle is apt to be clogged with water droplets and foreign matters such as scales and the like in the piping, resulting in a large variation of the negative pressure therein.
Thus, there arise the problems that the container may be misconceived as the so-called "swelled container" due to an insufficient negative pressure, or be subjected to a distinguished concave deformation and the like due to an excessive negative pressure, and lose its commodity value.
In the description the following type of container which can hold its own shape under no-load condition will be called a semi-rigid container, wherein a concavity generated in a wall portion such as a bottom wall portion due to a relatively low negative internal pressure (for example, -7 cmHg in gage pressure) will be of the degree that its commodity value will not be lost, but a wall portion such as a sidewall portion will be concavely deformed or collapse to the degree that its commodity value will be lost, due to a relatively high negative internal pressure (for example, -20 cmHg in gage pressure).
A container body used for the semi-rigid container will be called a semi-rigid container body in the description.
The "hot pack method" (c) can be applied only to liquid food products, and not to solid food products such as sausages or the like. Even for liquid food products, when applied to the semi-rigid container, there arises the problem that the concave deformation of the container accompanied by cooling after hermetic sealing may increase to the degree that its commodity value will be lost.
It is an object of the present invention to provide a method of making a hermetically sealed container placed with a product, which method is practicable by using a relatively inexpensive apparatus with a high productivity, applicable even to solid food products, and capable of controlling the negative internal pressure to a relatively small value and moreover with a small range of variation.
It is another object of the present invention to provide a method of making a hermetically sealed container, wherein oxygen in a space portion is reduced to a trace quantity.
It is still another object of the present invention to provide a method of making a hermetically sealed container which is placed with a product like foods or the like and has a wall portion that is elastically deformable concavely depending on a negative internal pressure, whose hermetic sealing performance has been confirmed, and whose negative internal pressure has been detected to be of the degree that the container has no danger of being misconceived as the "swelled container", even though Strecker degradation has generated, by using a relatively inexpensive apparatus with a high productivity.